I usually fly to and from South America (LIM) on American Airlines A300s. I always noticed that when the plane begins to move to the ramp (or just move), the engines make a spooling up sound (normal) and then make a sound as if they are spooling down (spooling up sound with a high pitch engine sound followed by a low pitched-hollow engine sound......But the thing is, the plane continues to move forward...even gain momentum!!). I sometimes fly on LAN Chiles 767s and noticed their engines don´t make that interesting sound. Hmmm.....Any thoughts anyone? I believe that American uses GE engines.

The A300 and the 767 are completely different aircraft with different characterisitcs. Airbus has been noted in the past as having weird engine noises especially on their A319/320/321 narrowbodies where the engine sound is described as a "yelping dog being blended."

The dog noise might be the hydraulic pumps (hear it all the time, and pax always get worried). As for the A300, I just flew on one last week (had been 12 years since my last ride), and the sound on take off was really something. High pitched whine, then deep....I love it!